Inductors are typically included among the discrete electronic components used in the circuit assemblies for electronic filters, such as notch filters and traps used in CATV systems. For these types of applications, it is particularly important that the inductors be tunable to the desired frequencies to be blocked or trapped by the filter.
It is known to use inductors which are free-floating, air-wound coils of wire having a predetermined number of turns. The inductance value of each coil is determined by the coil diameter, the number of turns, the distance between the wire turns, and the gage and length of the wire. Distortions present in th e coil also affect the inductance value.
The inductance value plays a role with respect to the overall circuit in that the coils are used to compensate for variations in other electrical components of the circuit, such as capacitive tolerances which can range from 2-5%. In that manner, inductor coils having a reliable natural frequency are desired to compensate for such variations. In order to obtain the desired natural frequency, the coils are subjected to a pre-alignment process wherein the coils are manually stretched such that each turn of the wire is separated from adjacent turns of the wire. The quality factor (Q) of the coil is highest when the diameter of the wire divided by the spacing between adjacent turns of the wire ranges from about 0.6 to 0.9.
There are several drawbacks associated with known inductors with respect to the structure, positioning, stretching and tuning thereof, and substantial room for improvement exists.
One problem is that numerous process steps are required to use air-wound coils in filter assemblies. First, an air-wound coil is positioned on a circuit board along with other discrete components for the circuit, and then the entire panel (i.e., circuit board array) is wave soldered. Next, the individual circuit boards are singulated from the panel. A screw guide is then added to each coil, and the coils are then manually stretched to a natural frequency to compensate for variations in the other electronic components. The circuit board is then positioned in a filter housing, which is subsequently potted before tuning slugs are inserted and screwed into the screw guides to manually tune each inductor.
Another problem is the human error factor associated with manually stretching the coils. That is, variations in human performance increase the difficulty of obtaining the desired pitch between adjacent wires when stretching the coils and often result in undesirable variations between coil units. For example, there can be a wide fluctuation in the actual Q (quality factor) of the coil due to the way the coil is stretched.
Additionally, excess flux used during the wave soldering step can migrate to the coils and effectively adhere the coil windings together. This adhesion makes it nearly impossible to stretch the coil to achieve the desired pitch during the coil stretching step of the pre-alignment process.
Yet another problem is that the coils themselves must be positioned on the circuit board without incurring distortions that affect the inductance value. For example, manual stretching and tuning may displace the coils laterally along the circuit board. This is undesirable because leaning coils will change the magnetic coupling therebetween and reduce the operating efficiency of the circuit. Further, any distortions or displacements along the length of the lead wire extending from the wound portion of the coil can also adversely affect performance and Q.
It would be desirable to provide tunable inductor coils that exhibit consistent Q and inductance values from unit to unit. It would also be desirable to provide inductor coils that do not need to be manually stretched for pre-alignment purposes, and which can structurally withstand handling during manufacturing.